Inhibition of Gli1 Enhances Remyelination Abilities of Endogenous Stem Cell Populations


Nerve cells, otherwise known as neurons, have long extensions called axons. Nerve impulses travel down these axons, away from the cell body towards another neuron that is connected to the neuron. The axons of some neurons are insulated with a special substance called myelin the layer of myelin that surrounds the axon. This “myelin sheath” acts as a protective covering composed of protein and lipids.

myelin_sheath

Axons can vary in length from anywhere to 1 millimeter or less to 1 meter. Sometimes, axons are bundled together to form nerves that transmit electrical nerve impulses across the body.

While myelin protects and insulates axons, it also enhances the speed at which nerve impulses are transmitted through the axon. Axons without myelin sheaths conduct nerve impulses continuously throughout the axon. However, myelinated axons have small, uncovered gaps in the myelin sheath called nodes of Ranvier. Myelinated axons can only conduct nerve impulse at the nodes of Ranvier. Consequently, the nerve impulse jumps from node to node, greatly increasing the speed of nerve impulse conduction.

nodes_of_ranvier

If myelin is damaged, the speed of nerve impulse transmission slows substantially. Multiple sclerosis is one example of a disease that causes systematic loss of the myelin sheath. Inflammatory demyelinating diseases also cause progressive damage and loss of the myelin sheath. Regenerating the myelin sheath in these patients is one of the goals of regenerative medicine.

A good deal of data tells us that endogenous remyelination does occur. Unfortunately, this process is overwhelmed by the degree of demyelination in these diseases. A stem cell population called the parenchymal oligodendrocyte progenitor cells and endogenous adult neural stem cells in the brain are known to remyelinate demyelinated axons.

The Salzer laboratory at the New York Neuroscience Institute examined the ability of a specific adult neural stem cell population to remyelinate axons. These stem cells expressed the transcription factor Gli1.

Salzer and his team showed that this subventricular zone-specific group of neural stem cells were efficiently recruited to demyelinated portions of the brain. This same neural stem cell population was never observed entering healthy axon tracts. This finding shows that these cells seem to specialize in making new myelin sheaths for damaged axon tracts.

Since these neural stem cells expressed Gli1, and since there are drugs that can inhibit Gli1 activity, Salzer’s group wanted to show that Gli1 was a necessary factor for neural stem cell activity. Surprisingly, differentiation of these neural stem cells into oligodendrocytes (which make myelin and remyelinate axons) is significantly enhanced by inhibition of Gli1.

A specific signaling pathway called the hedgehog pathway is known to activate Gli1 and other members of the Gli gene family. However, when the hedgehog pathway in these neural stem cells was completely inhibited, it did not have the same effect and Gli1 inhibition. This suggests that Gli1 is doing more than responding to the hedgehog pathway in these neural stem cells.

Salzer and his colleagues showed that Gli1 inhibition improved myelin deposition in an animal model of experimental autoimmune encephalomyelitis; an inflammatory demyelination disease. Thus, inhibition of Gli1 activity in this preclinical model system increase regeneration of the myelin sheath in demyelinated neurons.

This work elegantly showed that endogenous neural stem cells that can remyelinate axons are present and can be activated by inhibiting Gli1. Furthermore, this activation will nicely enhance the therapeutic capacity of these endogenous cells. This potentially identifies a new therapeutic avenue for the treatment of demyelinating disorders.

This work was published in Nature. 2015 Oct 15;526(7573):448-52. doi: 10.1038/nature14957.

Using Drugs to Stimulate your Own Stem Cells to Treat Multiple Sclerosis


Paul Tesar from Case Western Reserve in Cleveland. Ohio and his colleagues have discovered that two different drugs, miconazole and clobetasol, can reverse the symptoms of multiple sclerosis in laboratory animals. Furthermore, these drugs do so by stimulating the animals’ own native stem cell population that insulates nerves.

Multiple sclerosis (MS) is a member of the “demyelinating disorders.” The cause of MS remains unknown, but all of our available evidence strongly suggests that MS is an autoimmune disease in which the body’s immune system attacks its own tissues. In MS the immune system attacks and destroys myelin — the fatty substance that coats and protects nerve fibers in the brain and spinal cord. We can compare myelin to the insulation that surrounds electrical wires. When myelin is damaged, the nerve impulses that travel along that nerve may be slowed or blocked.

The myelin sheath is made by cells known as “oligodendrocytes,” and oligodendrocytes are derived from a stem cell population known as OPCs, which stands for oligodendrocyte progenitor cells. If this stem cell population could be stimulated, then perhaps the damaged myelin sheath could be repaired and the symptoms of MS ameliorated.

In a paper that appeared in the journal Nature (522, 2015 216-220), Tesar and the members of his research team, and his collaborators used a pluripotent mouse stem cell line and differentiated them into OPCs. Thyroid hormone is a known inducer of OPC differentiation. Therefore, Tesar and others screened a battery of drugs to determine if any of these compounds could induce OPC differentiation as cell as thyroid hormone. From this screen using cultured OPCs, two drugs, the antifungal drug miconazole and clobetasol, a corticosteroid of the glucocorticoid class, proved to do a better job of inducing OPC differentiation than thyroid hormone.

Was this an experimental artifact? Tesar and others devised an ingenious assay to measure the effectiveness of these two drugs. They used brain slices from fetal mice that were taken from animals whose brains had yet to synthesize myelin and applied OPCs to these slices with and without the drugs. With OPCs, no myelin was made because the OPCs did not receive any signal to differentiate into mature oligodendrocytes and synthesize myelin. However in the presence of either miconazole or clobetasol, the OPCs differentiated and successfully myelinated the brain slices.

Experiments in tissue culture are a great start, but do they demonstrate a biological reality within a live animal? To answer this question, Tesar and his crew injected laboratory mice with purified myelin. The immune systems of these mice generated a robust immune response against myelin that eroded the myelin sheath from their nerves. This condition mimics human MS and is called experimental autoimmune encephalitis, and it is an excellent model system for studying MS. When mice with experimental autoimmune encephalitis (EAE) were treated with either miconazole or clobetasol, the EAE mice showed a remarkable reversal of symptoms and a solid attenuation of demyelination. Tissue samples established that these reversals were due to increased OPC activity.

When the mechanisms of these drugs were examined in detail, it became clear that the two drugs worked through distinct biochemical mechanisms. Miconazole, for example, activated the mitogen-activated protein kinase (MAPK) pathway, but clobetasol worked through the glucocorticoid receptor signaling pathway. Both of these signaling pathways converge, however, to increase OPC differentiation.

Both miconazole and clobetasol are only approved for topical administration. However, the fact that these drugs can cross the blood-brain barrier and effect changes in the brain is very exciting. Furthermore, this work establishes the template for screening new compounds that might be efficacious in human patients.

In the meantime, human patients might benefit from a clinical trial that determines if the symptoms and neural damage caused by MS can be reversed by the administration of these drugs or derivatives of these drugs.

Spiking Stem Cells to Generate Myelin


Regenerating damaged nerve tissue represents a unique challenge for regenerative medicine. Nevertheless, some experiments have shown that it is possible to regenerate the myelin sheath that surrounds particular nerves.

Myelin is a fatty, insulating sheath that surrounds particular nerves and accelerates the transmission of nerve impulses. The myelin sheath also helps neurons survive, and the myelin sheath is attacked and removed in multiple sclerosis, a genetic disease called Charcot-Marie-Tooth disease, and spinal cord injuries. Being able to regenerate the myelin sheath is an essential goal of regenerative medicine.

Fortunately, a new study from a team of UC Davis (my alma mater) scientists have brought this goal one step closer. Wenbig Deng, principal investigator of this study and associate professor of biochemistry and molecular medicine, said, “Our findings represent an important conceptual advance in stem cell research. We have bioengineered the first generation of myelin-producing cells with superior regenerative capacity.”

The brain contains two main cell types; neurons and glial cells. Neurons make and transmit nerve impulses whereas glial cells support, nourish and protect neurons. One particular subtype of glial cells, oligodendrocytes, make the myelin sheath that surrounds the axons of many neurons. Deng and his group developed a novel protocol to induce embryonic stem cells (ESCs) to differentiate into oligodendrocyte precursor cells or OPCs. Even though other researchers have made oligodenrocytes from ESCs, Deng’s method results in purer populations of OPCs than any other available method.

Making OPCs from ESCs is one thing, but can these laboratory OPCs do everything native can do? When Deng and his team tested the electrophysiological properties of their laboratory-made OPCs, they discovered that their cells lacked an important component; they did not express sodium channels. When the lab-made OPCs were genetically engineered to express sodium channels, they generated the characteristic electrical spikes that are common to native OPCs. According to Deng, this is the first time anyone has made OPCs in the laboratory with spiking properties. Is this significant?

Deng and his colleagues compared the spiking OPCs to non-spiking OPCs in the laboratory. Not only did the spiking OPCs communicate with neurons, but they also did a better job of maturing into oligodentrocytes.

Transplantation of these two OPC populations into the spinal cord and brains of mice that are genetically unable to produce myelin also showed differences. Both types of OPCs were able to mature into oligodendrocytes and produce myelin sheaths, but only the spiking OPCs had the ability to produce longer and thicker myelin sheaths.

Said Deng, “We actually developed ‘super cells’ with an even greater capacity to spike than natural cells. This appears to give them an edge for maturing into oligodendrocytes and producing better myelin.

Human neural tissue has a poor capacity to regenerate and even though OPCs are present, they do not regenerate tissue effectively when disease or injury damages the myelin sheath. Deng believes that replacing glial cells with the enhanced spiking OPCs to treat injuries and diseases has the potential to be a better strategy than replacing neurons, since neurons are so problematic to work with in the laboratory. Instead providing the proper structure and environment for neurons to live might be the best approach to regenerate healthy neural tissue. Deng also said that many diverse conditions that have not been traditionally considered to be myelin-based diseases (schizophrenia, epilepsy, and amyotrophic lateral sclerosis) are actually now recognized to involve defective myelin.

On that one, I think Deng is dreaming. ALS is caused by the death of motor neurons due to mechanisms that are intrinsic to the neurons themselves. Giving them all the myelin in the world in not going to help them. Also, OPCs made from ESCs will be rejected out of hand by the immune system if they are used to regenerate myelin in the peripheral nervous system. The only hope is to keep them in the central nervous system, but even there, any immune response in the brain will be fatal to the OPCs. This needs to be tested with iPSCs before it can be considered for clinical purposes.

FDA Approves the First Stem Cell Clinical Trial for Multiple Sclerosis


The Tirsch Multiple Sclerosis (MS) Research Center of New York has received Investigational New Drug (IND) approval from the Food and Drug Administration to launch a Phase I trial that uses a patient’s own neural stem cells to treat MS.

MS is a chronic disease that results when a patient’s own immune system attacks the myelin insulation that covers many nerves. This damages the myelin sheath and causes degeneration of the nervous system. Some 2.1 million people worldwide are afflicted with MS.

“To my knowledge, this is the first FDA-approved stem cells trial in the United States to investigate direct injection of stem cells into the cerebrospinal fluid of MS patients, and represents an exciting advance in MS research and treatment,” said Saud A. Sadiq, senior research scientist at Tisch and the study’s principal investigator.

The groundbreaking study will evaluate the safety of using stem cells harvested from the patient’s own bone marrow. Once harvested, these stem cells will be injected into the cerebrospinal fluid that surrounds the spinal cord in 20 participants who meet the inclusion criteria for this trial.

Since this is a phase 1 study, it is an open safety and tolerability study. The Tisch MS Research Center and affiliated International Multiple Sclerosis Management Practice (IMSMP) will host all the activities associated with this study.

The clinical application of autologous neural precursors in MS is the culmination of a decade of stem cell research headed by Sadiq and his colleague Violaine Harris, a research scientist at Tisch.

Preclinical testing found that the injection of these cells seems to decrease inflammation in the brain and may also promote myelin repair and neuroprotection.  In a 2012 publication in the Journal of the Neurological Sciences, Harris and others showed that mesenchymal stem cell-derived neural progenitor cells could promote repair and recovery after intrathecal injection into mice with EAE (experimental autoimmune encephalitis), which is a MS-like disease in mice.  They were able to ascertain that intrathecal injection of mesenchymal stem cell-derived neural progenitor cells significantly correlated with reduced immune cell infiltration in the brain, reduced area of demyelination, and increased number of neural progenitor cells in EAE mice.  This successful preclinical study was the impetus for this clinical trial.

Sadiq said, “This study exemplifies the Tisch MS Research Center’s dedication to translational research and provides a hope that established disability may be reversed in MS.” All study participants will undergo a single bone marrow collection procedure, from which mesenchymal stem cell-derived neural progenitor cells (MSC-NPs) will be isolated. expanded, and tested prior to injection.

All patients will receive three rounds of injections at three-month intervals. Safety and efficacy parameters will be evaluated in all trial participants throughout their regular visits with their attending physicians.